The reaction of nucleophiles with polyvinylchloride (PVC) is known. (Ma, S. C. et al, Anal. Chem. 60 (1988) 2293; Chenh, P. et al, Nat'l. Sci. Council Monthly, ROC (1988) 8(4) (1988) 313; Biswas, Mukul et al, Indian J. of Technol. 28 (1990) 111) However, it is not common to derivatize PVC for most applications due to the fragility of the polymer and the relative difficulty of the substitution reaction. Elevated temperatures can lead to dehydrochlorination, which is autocatalytic. The addition of nucleophiles can also catalyze this process. The resultant polymer degradation is also accelerated by light, especially when it is in solution and at elevated temperatures. The derivatization of PVC by reaction with mines is normally carried out at high temperatures (Ma; Chenh; Biswas) and/or in a nonsolvent, such as methanol (Ma; Chenh). The result, in the first case, is a PVC which is severely degraded, and, in the second case, a polymer which is only partially derivatized. In the extreme worst case, the nucleophile is simply entrapped in the polymer matrix, giving rise to an apparent substitution reaction.
PVC is used in a number of applications in which it is desirable to have adhesion between the PVC and some substrate. Generally, the adhesion is achieved by treatment of the surface in question with a silane, such as dimethyldichlorosilane. (Harrison, D. J., J. Electroanal. Chem. 202 (1986) 75; Moody, G. J. et al, Analyst 113 (1988) 1703; Petrarch Systems, "Silicon Compounds, Register and Review" (1987) 54) In such a case, the adhesion is due to hydrophobic interactions, not the formation of covalent chemical bonds. The adhesion can be somewhat improved by the use of a silane which can form hydrogen bonds with the PVC, as these interactions are stronger than simple hydrophobic interaction.
One way to get around the fairly poor adhesion obtained by these methods is to heat the PVC extensively, in the presence of a binding agent, after it has been applied to the substrate. (Chakrabarti, S. et al, U.S. Pat. No. 4,341,686) Again, this leads to degradative dehydrochlorination of the polymer. Also, the material obtained by this method is not well-defined, nor is it necessarily crosslinked throughout the polymer layer.
One area where PVC adhesion is of particular importance is in the field of ion selective electrodes (hereafter referred to as ISEs). An integral part of these electrodes are the membranes which contain the ion selective components. These membranes are most often made from highly plasticized PVC, indeed, the plasticizer content is nearly always greater than the PVC content. This fact is important, since, due to the oily nature of the plasticizers, adhesion of this membrane to any surface is extremely difficult to achieve.
Failure of the membrane to adhere to the electrode substrate can lead to shorting or shunt formation between the working electrode and the reference electrode when the entire electrode is immersed in aqueous analyte.
Another disadvantage of the highly plasticized formulations used in ISEs is their ability to flow over long periods of time. The membranes are actually a viscous liquid; storage of the electrodes can result in membranes which are not of uniform thickness. The fact that the novel polymer crosslinks upon curing prevents flow in the resulting membrane.